Electric resistance welded steel pipe for producing hollow stabilizer, hollow stabilizer, and production methods for same

ABSTRACT

There are provided an electric resistance welded steel pipe for producing a high strength hollow stabilizer excellent in fatigue resistance and a high strength hollow stabilizer. In an electric resistance welded steel pipe (5) for producing a hollow stabilizer, an internal weld bead cut portion (30) has a three-peak shape and a depth (H) of a trough portion (30a) of the three-peak shape is 0.3 mm or less and an angle (θ) formed by a central portion in the circumferential direction of the trough portion (30a) and the top of right and left peak portions (30b, 30c) located on both the right and left sides of the trough portion (30a) is 160° or more and less than 180°.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2019/009811, filedMar. 11, 2019 which claims priority to Japanese Patent Application No.2018-065875, filed Mar. 29, 2018, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE INVENTION

The present invention is a technology relating to a hollow stabilizer.

Herein, in this specification, the Vickers hardness is expressed byaverage hardness in the thickness direction. “High strength” indicates acase where the strength is 400 HV or more and preferably 450 HV or morein terms of the Vickers hardness.

BACKGROUND OF THE INVENTION

In usual, an automobile has a stabilizer in order to reduce the rollingof an automobile body in cornering or to keep the traveling stability inhigh speed traveling. Recently, it is common to use a hollow stabilizercontaining a steel pipe as the stabilizer in order to reduce the weightof the automobile body.

Such a hollow stabilizer commonly contains a seamless steel pipe or anelectric resistance welded steel pipe produced by electric resistancewelding as a raw material. Such a steel pipe is molded into a targetstabilizer shape by cold rolling, and then subjected to thermal refiningtreatment, such as quenching or quenching and tempering, to be made intoa product. In particular, the electric resistance welded steel pipe isrelatively inexpensive and excellent in dimensional accuracy, andtherefore has been widely used as a raw material for producing thehollow stabilizer.

As the inventions of conventional hollow stabilizers, the inventionsdescribed in PTLS 1 and 2 are mentioned, for example.

More specifically, PTL 1 describes a production method for a hollowstabilizer excellent in fatigue resistance.

According to PTL 1, a welded steel pipe having a composition containing,in terms of % by mass, C: 0.2 to 0.38%, Si: 0.35% or less, Mn: 0.3 to1.5%, Al: 0.1% or less, Ti: 0.005 to 0.1%, and B: 0.0005 to 0.005% isused as a raw material and the raw material is subjected to heattreatment including heating the raw material to a temperature in therange of 800 to 1000° C., and then subjected to reduction rolling with acumulative reduction ratio: 40% or more at a rolling temperature: 600 to850° C. to be made into a steel pipe for producing a hollow stabilizer.Thereafter, according to PTL 1, the steel pipe is sequentially subjectedto a molding step of molding the steel pipe into a stabilizer shape bycold bending and a heat treatment step of applying quenching treatmentand tempering treatment to the steel pipe to form a hollow stabilizer.PTL 1 describes that the production method can improve the fatigueresistance by an inexpensive method.

According to PTL 2, a steel sheet having a composition containing, interms of % by mass, C: 0.15 to 0.40%, Si: 0.05 to 0.50%, Mn: 0.30 to2.00%, Al: 0.01 to 0.10%, Ti: 0.001 to 0.04%, B: 0.0005 to 0.0050%, andN: 0.0010 to 0.0100% and containing Ti, N to satisfy (N/14)<(Ti/47.9) ismade into a substantially cylindrical-shaped open pipe, end portions ofthe open pipe are butted against each other, and then electricresistance welding is performed to the end portions while adjusting heatinput such that the bond width is 30 to 65 μm by high frequencyresistance welding to form an electric resistance welded pipe. Then,according to PTL 2, the electric resistance welded pipe is heated to atemperature equal to or higher than a Ac₃ transformation point, and thensubjected to stretch reducing with a draft equal to or higher than(1-25/Bond width before stretch reducing (μm)×100% in terms of an outerdiameter ratio, so that the bond width is set to 25 or less. PTL 2describes producing an electric resistance welded steel pipe for heattreatment excellent in flatness by the treatment above. PTL 2 describesthat the produced electric resistance welded steel pipe is suitable forthe use of being subjected to quenching treatment, e.g., a hollowstabilizer, and that the reduced-carbon layer width of an electricresistance weld zone is narrow, and therefore, even when quenchingtreatment by rapid and short-time heating is applied thereto, areduction in the quenching hardness of the electric resistance weld zonecan be suppressed, so that a hollow stabilizer excellent in durabilitycan be obtained.

PATENT LITERATURE

PTL 1: JP 2005-076047 A

PTL 2: JP 2008-208417 A

SUMMARY OF THE INVENTION

When hot reducing work is applied to a steel pipe as in the technologiesdescribed in PTLS 1 and 2, irregularities accompanying pipe innersurface reducing are generated in the steel pipe. Particularly in theelectric resistance weld zone, bead cutting treatment is applied to aninternal weld zone in a base pipe stage before the hot reducing work sothat an internal weld bead cut portion is formed. The weld bead cutshape in the base pipe stage is affected by a difference between thehardness of a weld bond zone and the hardness of a peripheral portionthereof. Therefore, the shape of the internal weld bead cut portion isdeformed into a complicated three-peak shape by the hot reducing work.Then, the present inventors have conducted an examination to obtain afinding that a portion having the three-peak shape of the internal weldbead cut portion serves as a stress concentration portion when atorsional stress is given to a stabilizer to cause a reduction in thefatigue characteristics depending on the situation. The reduction in thefatigue characteristics due to the stress concentration becomesremarkable particularly in the case of a high strength stabilizer.

Furthermore, with an increase in the strength of the stabilizer and witha reduction in the pipe thickness, a stress on the pipe outer surfaceside and a stress on the pipe inner surface side become closer to eachother with respect to the stress generated when the stabilizer istwisted. Therefore, the stabilizer is likely to be affected by thesurface properties or the surface shape of the pipe inner surface side,which increases the probability of the occurrence of a crack with thepipe inner surface side as the starting point. The occurrence of a crackis a major issue of a pipe produced by the hot reducing work. Colddrawing (cold drawing work) smoothes the inner surface shape butinvolves an increase in the production cost.

Aspects of the present invention have been made focusing on theabove-described points. It is an object according to aspects of thepresent invention to provide an electric resistance welded steel pipefor producing a hollow stabilizer excellent in fatigue resistance andhaving high strength and a hollow stabilizer excellent in fatigueresistance and having high strength.

The present inventors have examined the influence of the shape of aninternal weld bead zone on the fatigue characteristics of a highstrength hollow stabilizer. As a result, the present inventors haveobtained a finding that it is important, with respect to the fatigue, tocontrol a depth H of a thin portion (trough portion) of a three-peakshape and an angle θ formed by the thin portion (trough portion) andright and left peak portions containing central thick portions of thethree-peak shape in the internal weld bead zone within a specific range.

Aspects of the present invention have been accomplished by furtheradding an examination based on the finding.

In order to solve the problems, in an electric resistance welded steelpipe for producing a hollow stabilizer of one aspect of the presentinvention, an internal weld bead cut portion has a three-peak shape, adepth H of a trough portion of the three-peak shape is 0.3 mm or less,and an angle θ formed by a central portion in the circumferentialdirection of the trough portion and the top of right and left peakportions located on both the right and left sides of the trough portionis 160° or more and less than 180°.

One aspect of the present invention is a hollow stabilizer containingthe electric resistance welded steel pipe for producing a hollowstabilizer of one aspect described above and having Vickers hardness of400 HV or more and less than 580 HV.

A production method for a hollow stabilizer of one aspect of the presentinvention includes applying cold bending to the electric resistancewelded steel pipe for producing a hollow stabilizer of one aspectdescribed above, and then applying heat treatment including quenchingtreatment or quenching and tempering treatment thereto so that thehardness after the heat treatment is set to 400 HV or more and less than580 HV in terms of the Vickers hardness.

A production method for an electric resistance welded steel pipe forproducing a hollow stabilizer of one aspect of the present inventionincludes a first step of molding a steel sheet into a cylindrical shapeby cold molding to form an open pipe, butting end portions in the widthdirection of the open pipe against each other, and performing electricresistance welding to the end portions to form an electric resistancewelded pipe, a second step of heating the electric resistance weldedpipe to a temperature of 850° C. or more and 1000° C. or less, and thenapplying hot stretch reducing thereto at a rolling temperature: 650° C.or more and 1000° C. or less and at a cumulative reduction ratio: 30% ormore and 90% or less, and a third step of adjusting an internal weldbead cut portion having an outline shape of a three-peak shape along thecircumferential direction in which a trough portion with a reducedthickness is formed on each of both the right and left sides across aweld zone such that a depth H of the trough portion is 0.3 mm or lessand an angle θ formed by a central portion in the circumferentialdirection of the trough portion and the top of right and left peakportions located on both the right and left sides of the trough portionis 160° or more and less than 180°.

Aspects of the present invention can provide an electric resistancewelded steel pipe suitable for the production of a hollow stabilizerexcellent in fatigue resistance. In particular, the electric resistancewelded steel pipe is suitable for a high strength hollow stabilizerexcellent in fatigue resistance.

For example, aspects of the present invention can simply produce a highstrength hollow stabilizer having hardness of 400 HV or more and holdingexcellent corrosion fatigue resistance, and thus can exhibit remarkableindustrial effects. Moreover, aspects of the present invention can alsoexhibit an effect that, even when the hardness is further increased,e.g., hardness of 450 HV or more, a reduction in fatigue characteristicsis not observed and a contribution to a further thickness reduction of astabilizer can be made, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram explaining a production method for an electricresistance welded steel pipe for producing a hollow stabilizer accordingto an embodiment based on the present invention;

FIG. 2 is a conceptual diagram explaining the production method for anelectric resistance welded steel pipe for producing a hollow stabilizeraccording to the embodiment based on the present invention;

FIG. 3 is a figure explaining the shape of an internal bead cut portionin an electric resistance welded pipe produced in a first step;

FIG. 4 is a figure illustrating the shape, a depth H of a troughportion, and an angle θ of the internal bead cut portion having athree-peak shape generated by treatment by a hot reducing treatmentunit; and

FIG. 5 is a figure illustrating an example of the shape of a hollowstabilizer.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An embodiment of the present invention will now be described withreference to the drawings.

Herein, the drawings are schematic and the relationship between thethickness and the plane dimension, the distance between components, andthe like are different from the actual relationship, distance, and thelike. An embodiment described below describes the configuration forspecifying the technological idea according to aspects of the presentinvention as an example. The technological idea according to aspects ofthe present invention does not limit the shape, structure, and the likeof constituent components to the following shape, structure, and thelike. The technological idea according to aspects of the presentinvention can be variously altered in the technological scope specifiedby Claims described in Claims.

This embodiment relates to an electric resistance welded steel pipehaving an internal weld bead cut portion having an outline shape of athree-peak shape suitable as a steel pipe to be worked into a hollowstabilizer for use in an automobile and other vehicles and a hollowstabilizer produced from the electric resistance welded steel pipe. Inparticular, this embodiment is a technology suitable for producing anelectric resistance welded steel pipe capable of producing a hollowstabilizer having high strength and improved fatigue characteristics ora high strength hollow stabilizer.

This embodiment is a technology particularly suitable for a case where(Thickness/Outer diameter) as the dimension of the high strength hollowstabilizer is 20% or less.

<Production Method for Electric Resistance Welded Steel Pipe 5 forProducing Hollow Stabilizer>

A production method for an electric resistance welded steel pipe 5 forproducing a hollow stabilizer of this embodiment has a first step 1 anda second step 3 as illustrated in FIG. 1.

(First Step 1)

The first step 1 includes producing an open pipe molding unit 1A ofmolding a steel sheet into a substantially cylindrical shape (tubularshape) by cold molding to form an open pipe, and producing an electricresistance welding treatment unit 1B of butting end portions in thewidth direction of the open pipe against each other, and performingelectric resistance welding to the end portions to form an electricresistance welded pipe 2 as illustrated in FIGS. 1 and 2.

As illustrated in FIG. 2, the open pipe molding unit 1A performstreatment of continuously performing cold molding with a plurality ofrolls to form the open pipe of the substantially cylindrical shape, forexample.

The electric resistance welding treatment unit 1B performs treatment ofbutting the end portions in the width direction of the open pipe againsteach other with a squeeze roll, and performing electric resistancewelding to the end portions by high frequency resistance welding,induction heating, and the like to form the electric resistance weldedpipe 2 (electric resistance welded steel pipe) of a predetermineddimension, for example.

As the steel sheet to be used, FIG. 2 illustrates a case where the steelsheet is in a state of a steel strip. The steel sheet is preferably ahot-rolled steel sheet produced by hot-rolling.

The first step 1 further includes producing a bead cutting treatmentunit 1C of cutting a weld bead generated on each of the pipe outersurface side and the pipe inner surface side by electric resistancewelding as post-treatment of the electric resistance welding treatmentunit 1B. The bead cutting treatment unit 1C performs treatment ofcontinuously cutting weld beads 11 illustrated in FIG. 3 by a cuttingtool. This cutting forms an internal weld bead cut portion with anoutline having a circular arc-shaped cross section.

More specifically, the treatment of the bead cutting treatment unit 1Cforms the internal weld bead cut portion as indicated by an alternatelong and short dashed line 20 in FIG. 3 on the pipe inner surface. Inthis state, the shape of the internal weld bead cut portion has onetrough shape in which a weld zone is cut into the circular arc-shapedcross section with a weld line 12 as a central portion (see thealternate long and short dashed line 20 of FIG. 3).

(Second Step 3)

The second step 3 includes producing a heat treatment unit 3A of heatingthe electric resistance welded pipe 2 produced in the first step 1, andproducing a hot stretch reducing treatment unit 3B of applying hotstretch reducing to the heated electric resistance welded pipe 2 to formthe electric resistance welded steel pipe 5 as illustrated in FIGS. 1and 2. A reference numeral 3C designates warm cutting treatment.

The heat treatment unit 3A performs treatment of heating the electricresistance welded pipe 2. The heating temperature is preferably set to850° C. or more and 1000° C. or less. When the heating temperature isless than 850° C., desired weld zone toughness cannot be secured in somecases. On the other hand, at a high temperature exceeding 1000° C.,surface decarburization becomes remarkable, so that the surfaceproperties decrease in some cases.

The hot stretch reducing treatment unit 3B performs treatment ofapplying hot stretch reducing. The conditions of the hot stretchreducing are preferably the rolling temperature: 650° C. or more and1000° C. or less and the cumulative reduction ratio: 30% or more and 90%or less. The cumulative reduction ratio: 30% or more and 80% or less ismore preferable.

Herein, when the rolling temperature in the hot stretch reducing is lessthan 650° C., the workability decreases, which makes the molding into adesired stabilizer shape difficult in some cases.

The cumulative reduction ratio is a reduction ratio generated before andafter the hot stretch reducing treatment.

When the hot stretch reducing is applied, the thickness of a weld bondzone located in a central portion of the trough shape increases and thethickness of both the right and left sides of the trough shape alsoincreases due to a difference between the hardness of the weld bond zoneand the hardness of a peripheral portion thereof. Thus, one trough shapeis divided into two trough portions 30 a and three peaks due to thethickness increase are formed in an internal weld bead cut portion 30 asillustrated in FIG. 4. As a result, the shape of the internal weld beadcut portion 30 is changed into a three-peak shape.

This embodiment has a third step 4 of leveling the three-peak shape ofthe internal weld bead cut portion 30. In this embodiment, the thirdstep 4 is carried out in synchronization with the hot stretch reducingtreatment unit 3B of the second step 3.

The third step 4 is a step of adjusting the internal weld bead cutportion 30 having an outline shape of the three-peak shape along thecircumferential direction in which the trough portion 30 a with areduced thickness is formed on each of both the right and left sidesacross the weld line 12 such that a depth H of the trough portion 30 ais 0.3 mm or less and an angle θ formed by a central portion in thecircumferential direction of the trough portion 30 a and the top ofright and left peak portions located on both the right and left sides ofthe trough portion 30 a is 160° or more and less than 180° after the hotstretch reducing. The central portion in the circumferential directionof the trough portion 30 a serves as the substantially deepest position.

The third step 4 is performed by adjustment treatment of forming thesteel pipe 5 into a thinner gauged steel sheet, for example. Forexample, the third step 4 is treatment of forming a steel pipe into athinner gauged steel sheet such that a thickness reduction ratio is 0%or more and 10% or less and preferably 3% or more and 10% or less beforeand after the stretch reducing. The thickness reduction ratio isspecified by (Thickness before reducing−Thickness afterreducing)/Thickness before reducing×100(%).

Herein, by properly adjusting the cumulative reduction ratio and thethickness reduction ratio within the range where the cumulativereduction ratio is 30% or more and 90% or less and the thicknessreduction ratio before and after the stretch reducing is 0% or more and10% or less, the shape of the internal weld bead cut portion 30satisfying the depth H range and the angle θ range above can be molded.

More specifically, the adjustment of the thickness reduction ratiobefore and after stretch reducing rolling in accordance with thecumulative reduction ratio in the hot stretch reducing enables theadjustment such that the depth H of the trough portion 30 a is 0.3 mm orless and the angle θ formed by the central portion in thecircumferential direction of the trough portion 30 a and the top of theright and left peak portions located on both the right and left sides ofthe trough portion 30 a is 160° or more and less than 180°.

The depth of the trough portion 30 a is a depth with respect to thestraight line connecting the peak portions 30 b, 30 c on both the rightand left sides as illustrated in FIG. 4.

Herein, when the cumulative reduction ratio is less than 30%, thecumulative reduction ratio is not suitable for the size as a steel pipefor producing a stabilizer. When the cumulative reduction ratio exceeds90%, irregularities of a raw material of the pipe inner surface increaseeven when the stretch in the longitudinal direction of the pipe isincreased, and therefore a desired internal weld bead shape is notobtained.

When the thickness reduction ratio is less than 0%, i.e., when thethickness is increased, irregularities of the raw material of the pipeinner surface increase, which makes it difficult to improve the internalweld bead shape. On the other hand, when the thickness reduction ratioexceeds 10%, the stretching force becomes excessive, and thus a fractureoccurs in the hot stretch reducing or a thin portion of the pipeinternal weld bead zone becomes remarkable, which leads to a reductionin the fatigue characteristics.

<Configuration of Electric Resistance Welded Steel Pipe 5 for ProducingHollow Stabilizer>

The electric resistance welded steel pipe 5 for producing a hollowstabilizer of this embodiment is produced by the production methoddescribed above, for example.

The electric resistance welded steel pipe 5 for producing a hollowstabilizer of this embodiment is the electric resistance welded steelpipe 5 which is a steel pipe for producing a hollow stabilizer and hasthe internal weld bead cut portion 30 having the outline shape of thethree-peak shape along the circumferential direction in which the troughportion 30 a with a reduced thickness is formed on each of both theright and left sides across the weld zone on the pipe inner surface. Inthe internal weld bead cut portion 30, the depth H of the trough portion30 a is 0.3 mm or less and the angle θ formed by the central portion inthe circumferential direction of the trough portion 30 a and the top ofright and left peak portions 30 b, 30 c located on both the right andleft sides of the trough portion 30 a is 160° or more and less than180°.

Steel materials configuring the electric resistance welded steel pipe 5are steel materials having Vickers hardness of 400 HV or more andpreferably 450 HV or more. In order to obtain a hollow stabilizer havingVickers hardness of 450 HV or more, steel materials preferably contain,in terms of % by mass, C: 0.20% or more and 0.40% or less, Si: 0.1% ormore and 1.0% or less, Mn: 0.1% or more and 2.0% or less, P: 0.100% orless, S: 0.010% or less, Al: 0.01% or more and 0.10% or less, Cr: 0.01%or more and 0.50% or less, Ti: 0.01% or more and 0.05% or less, B:0.0005% or more and 0.0050% or less, Ca: 0.0001% or more and 0.0050% orless, N: 0.0050% or less, and the balance of Fe and inevitableimpurities.

It is preferable for the steel materials configuring the electricresistance welded steel pipe 5 to further contain, in terms of % bymass, either or both of Cu: 0.05% or more and 1.00% or less and Ni:0.05% or more and 1.00% or less.

It is preferable for the steel materials configuring the electricresistance welded steel pipe 5 to further contain, in terms of % bymass, one or two or more kinds selected from Nb: 0.001% or more and0.005% or less, W: 0.001% or more and 0.050% or less, and V: 0.05% ormore and 0.50% or less.

It is preferable for the steel materials configuring the steel pipe tofurther contain, in terms of % by mass, REM: 0.001% or more and 0.020%or less.

(Reasons for Limiting Composition)

Next, reasons for limiting the composition of the electric resistancewelded steel pipe 5 are described. Hereinafter, % by mass is simplyexpressed by “%” unless otherwise particularly specified.

-   -   C: 0.20% or more and 0.40% or less

C is an element having actions of promoting the generation of martensitethrough an improvement of hardenability and increasing the strength(hardness) of steel by forming a solid solution and important forincreasing the strength of a hollow stabilizer. In this embodiment, inorder to set the hardness after quenching and tempering treatment to 400HV or more in terms of the Vickers hardness, the content needs to be0.20% or more.

On the other hand, when a large amount of C is contained so that thecontent exceeds 0.40%, the toughness after the quenching treatmentdecreases. Therefore, C was limited to the range of 0.20% or more and0.40% or less. C is preferably 0.22% or more. C is preferably 0.39% orless.

-   -   Si: 0.1% or more and 1.0% or less

Si acts as a deoxidant and also acts as a solid solution strengtheningelement. In order to obtain such effects, the Si content needs to be0.1% or more. On the other hand, when the Si content exceeds 1.0%, theelectric resistance weldability decreases. Therefore, Si was limited tothe range of 0.1% or more and 1.0% or less. Si is preferably 0.5% orless.

-   -   Mn: 0.1% or more and 2.0% or less

Mn is an element forming a solid solution to contribute to an increasein the strength of steel and improving the hardenability of steel. Inthis embodiment, the Mn content needs to be 0.1% or more in order tosecure desired high strength (high hardness). On the other hand, whenthe Mn content exceeds 2.0%, the toughness decreases and a risk of aquenching crack increases. Therefore, Mn was limited to the range of0.1% or more and 2.0% or less. Mn is preferably 0.5% or more. Mn ispreferably 1.8% or less.

-   -   P: 0.100% or less

P is an element present as an impurity and segregated in the grainboundary or the like to adversely affect weld crack properties andtoughness. Thus, it is necessary to decrease P to 0.100% or less as asubstance for hollow stabilizers. P is preferably 0.050% or less. SinceP is inevitably contained, the P content usually reaches 0.001% or more.

-   -   S: 0.010% or less

S is an element present as a sulfide-based inclusion in steel andreducing the hot workability, toughness, and fatigue resistance. Thus,it is necessary to decrease S to 0.010% or less as a substance forhollow stabilizers. S is preferably 0.005% or less. Since S isinevitably contained, the S content usually reaches 0.001% or more.

-   -   Al: 0.01% or more and 0.10% or less

Al acts as a deoxidant and is bonded to N to exhibit an effect ofsecuring the amount of solid solution B effective for an improvement ofhardenability. Moreover, Al is precipitated as AlN and has an action ofpreventing the coarsening of austenite grains in quenching and heating.In order to obtain such effects, the Al content needs to be 0.01% ormore. On the other hand, when a large amount of Al is contained so thatthe content exceeds 0.10%, the amount of oxide-based inclusionsincreases, which shortens the fatigue life. Therefore, Al was limited tothe range of 0.01% or more and 0.10% or less. Al is preferably 0.05% orless.

-   -   Cr: 0.01% or more and 0.50% or less

Cr is an element improving the hardenability of steel and contributingto an improvement of corrosion resistance. In order to obtain sucheffects, the content needs to be 0.01% or more. On the other hand, whenthe Cr content exceeds 0.50%, the electric resistance weldabilitydecreases. Therefore, Cr was limited to the range of 0.01% or more and0.50% or less. Cr is preferably 0.1% or more. Cr is preferably 0.3% orless.

-   -   Ti: 0.01% or more and 0.05% or less

Ti is bonded to N to exhibit an effect of securing the amount of solidsolution B effective for an improvement of hardenability. Moreover, Tiis precipitated as fine carbide to contribute to refining of austenitegrains in heat treatment, such as quenching, and contribute to animprovement of the fatigue resistance in a corrosion environment(corrosion fatigue resistance). In order to obtain such effects, the Ticontent needs to be 0.01% or more. On the other hand, when the contentexceeds Ti: 0.05%, a coarse titanium sulfide (TiS) is likely to beformed and the coarse titanium sulfide is likely to serve as thestarting point of a corrosion pit, so that the corrosion resistance andthe corrosion fatigue resistance decrease. Therefore, Ti was limited tothe range of 0.01% or more and 0.05% or less. Ti is preferably 0.04% orless.

-   -   B: 0.0005% or more and 0.0050% or less

B is an effective element improving the hardenability of steel at atrace amount. B has an action of strengthening the grain boundary andsuppresses the grain boundary embrittlement due to the segregation of P.In order to obtain such effects, the B content needs to be 0.0005% ormore. On the other hand, even when the B content exceeds 0.0050%, theeffects are saturated, which is economically disadvantageous. Therefore,B was limited to the range of 0.0005% or more and 0.005% or less. B ispreferably 0.0010% or more. B is preferably 0.0030% or less.

-   -   Ca: 0.0001% or more and 0.0050% or less

Ca is an element having an action of controlling the form ofsulfide-based inclusions into fine inclusions of a substantiallyspherical shape. Since coarse MnS particles have a risk of serving asthe starting point of a fatigue fracture, Ca is added in order tosuppress the generation of MnS. In order to obtain such effects, the Cacontent needs to be 0.0001% or more. On the other hand, when a largeamount Ca is contained so that the content exceeds 0.0050%, the amountof coarse CaS-based clusters excessively increases and the coarseCaS-based clusters conversely serve as the starting point of a fatiguecrack, so that the fatigue characteristics decrease. Therefore, Ca waslimited to the range of 0.0001% or more and 0.0050% or less. Ca ispreferably 0.0010% or more. Ca is preferably 0.0030% or less.

-   -   N: 0.0050% or less

N is inevitably contained as an impurity. N is bonded to nitride formingelements in steel to contribute to the suppression of the coarsening ofcrystal grains and further an increase in the strength after tempering.However, the N content exceeding 0.0050% reduces the toughness of a weldzone. Therefore, N was limited to 0.0050% or less. N is preferably0.0010% or less. N is more preferably 0.0003% or less. Since N isinevitably contained, the content usually reaches 0.0001% or more.

-   -   One or two kinds selected from Cu: 0.05% or more and 1.00% or        less and Ni: 0.05% or more and 1.00% or less

Both Cu and Ni are elements improving the hardenability and improvingthe corrosion resistance and can be selected and contained as necessary.In order to obtain such effects, the contents of Cu: 0.05% or more andNi: 0.05% or more are needed. On the other hand, both Cu and Ni areexpensive elements, and thus, when the contents exceed Cu: 1.00% and Ni:1.00%, a material cost increase is caused. Therefore, when Cu, Ni arecontained, it is preferable to limit the contents to Cu: 1.00% or lessand Ni: 1.00% or less. The contents are preferably limited to Cu: 0.05%or more and Ni: 0.05% or more. Cu: 0.10% or more and Ni: 0.10% or moreare more preferable. Cu: 0.50% or less and Ni: 0.50% or less are morepreferable.

-   -   One or two or more kinds selected from Nb: 0.001% or more and        0.050% or less, W: 0.001% or more and 0.050% or less, and V:        0.05% or more and 0.50% or less

Nb, W, and V are all elements forming fine carbides to contribute to anincrease in the strength (hardness). One or two or more kinds thereofcan be selected and contained as necessary. In order to obtain such aneffect, the contents of Nb: 0.001% or more, W: 0.01% or more, and V:0.05% or more are needed. On the other hand, even when the contentsexceed Nb: 0.050%, W: 0.050%, and V: 0.50%, the effects are saturated,and thus the effects matching the contents cannot be expected, which iseconomically disadvantageous. The carbides are likely to be coarsened toadversely affect the toughness. Therefore, when Nb, W, and V arecontained, it is preferable to limit the contents to Nb: 0.050% or less,W: 0.050% or less, and V: 0.50% or less. Nb: 0.010% or more, W: 0.010%or more, and V: 0.10% or more are more preferable. Nb: 0.030% or less,W: 0.030% or less, and V: 0.30% or less are more preferable.

-   -   REM: 0.001% or more and 0.020% or less

REM is an element having an action of controlling the form ofsulfide-based inclusions into fine inclusions having a substantiallyspherical shape as with Ca. In this embodiment, it is preferable tocontain 0.001% or more of REM from the viewpoint of complementing theaction of Ca. On the other hand, when the REM content exceeds 0.020%,the amount of the inclusions excessively increases and the inclusionsconversely serve as the starting point of a fatigue crack, so that thefatigue characteristics decrease. Therefore, when REM is contained, REMis preferably limited to 0.020% or less. REM is more preferably 0.001%or more. REM is more preferably 0.010% or less.

<Effects of Electric Resistance Welded Steel Pipe 5 for Producing HollowStabilizer>

In the electric resistance welded steel pipe 5 for producing a hollowstabilizer, the internal weld bead cut portion 30 is formed so that thedepth H of the trough portion 30 a is 0.3 mm or less and the angle θformed by the central portion in the circumferential direction of thetrough portion 30 a and the top of the right and left peak portions 30b, 30 c located on both the right and left sides of the trough portion30 a is 160° or more and less than 180°.

Even when a hollow stabilizer with reduced local stress concentration inthe weld bead zone of the pipe inner surface, high strength, and areduced thickness is produced by applying this configuration, highfatigue characteristics can be obtained (see Example described later).

More specifically, the electric resistance welded steel pipe 5 forproducing a hollow stabilizer of this embodiment is the electricresistance welded steel pipe 5 which is a steel pipe for producing ahollow stabilizer and has, on the pipe inner surface, the internal weldbead cut portion 30 having the outline shape of the three-peak shapealong the circumferential direction in which the trough portion 30 awith a reduced thickness is formed on each of both the right and leftsides across the weld zone, in which the internal weld bead cut portion30 has the depth H of the trough portion 30 a of 0.3 mm or less and theangle θ formed by the central portion in the circumferential directionof the trough portion 30 a and the top of the right and left peakportions 30 b, 30 c located on both the right and left sides of thetrough portion 30 a of 160° or more and less than 180°.

It is found that this configuration can improve the fatiguecharacteristics of a hollow stabilizer produced using the electricresistance welded steel pipe 5 for producing a hollow stabilizer.

At this time, steel materials configuring the steel pipe may contain, interms of % by mass, C: 0.20% or more and 0.40% or less, Si: 0.1% or moreand 1.0% or less, Mn: 0.1% or more and 2.0% or less, P: 0.100% or less,S: 0.010% or less, Al: 0.01% or more and 0.10% or less, Cr: 0.01% ormore and 0.50% or less, Ti: 0.01% or more and 0.05% or less, B: 0.0005%or more and 0.0050% or less, Ca: 0.0001% or more and 0.0050% or less, N:0.0050% or less, and the balance of Fe and inevitable impurities.

By adopting this configuration, the hardness of the produced hollowstabilizer can be certainly set to 400 HV or more and less than 580 HV,and thus a high strength hollow stabilizer excellent in fatiguecharacteristics can be provided.

<Hollow Stabilizer>

A hollow stabilizer 40 of this embodiment is a hollow stabilizer whichis obtained by working the electric resistance welded steel pipe 5 forproducing a hollow stabilizer into a shape illustrated in FIG. 5 so thatthe Vickers hardness is 400 HV or more and less than 580 HV.

The hollow stabilizer has, for example, a molding step of applying coldbending to the electric resistance welded steel pipe 5 for producing ahollow stabilizer described above and a heat treatment step of applyingheat treatment including quenching treatment or quenching and temperingtreatment thereto after the molding step and is produced so that thehardness after the heat treatment is adjusted to 400 HV or more and lessthan 580 HV in terms of the Vickers hardness by the heat treatment ofthe heat treatment step. Furthermore, shot blasting treatment may beapplied to the pipe inner surface, the pipe outer surface, or the pipeinner and outer surfaces.

The molding step molds the electric resistance welded steel pipe 5 intoa target stabilizer shape. As molding methods, all known molding methodsare applicable. The treatment in the molding step is preferably coldbending from the viewpoint of suppressing the surface decarburization.In the cold bending, rotary draw bending, press bending, and the likecan be illustrated.

The heat treatment step includes quenching treatment or quenching andtempering treatment.

The quenching treatment is preferably treatment including heating theelectric resistance welded steel pipe 5 to a temperature equal to orhigher than the Ac₃ transformation point, preferably a temperature of1100° C. or less, holding the steel pipe at the temperature for apredetermined time, preferably 1 second or more, placing the electricresistance welded steel pipe 5 in a quenching vessel, and then rapidlycooling the electric resistance welded steel pipe 5 at a cooling rate of10° C./s or more and 100° C./s or less, for example. Thus, the hollowstabilizer 40 of this embodiment can possess high strength and hightoughness.

When the quenching heating temperature exceeds 1100° C. to be a hightemperature, austenite grains are coarsened. The heating is preferablyelectrical heating from the viewpoint of the suppression of surfacedecarburization and the productivity. A refrigerant of a quenchingvessel is preferably water, quenching oil, or a concentration-adjustedmixed liquid of water and a polymer.

It is preferable that, after the quenching treatment, temperingtreatment is further applied. In the tempering treatment, the temperingtemperature is preferably adjusted in accordance with desired hardness.The tempering temperature is suitably 200° C. or more and 450° C. orless. By applying the tempering treatment, the toughness is markedlyimproved.

It is needless to say that the application of common shot blastingtreatment to the pipe inner surface, the pipe outer surface, or the pipeinner and outer surfaces after the heat treatment is preferable for animprovement of the fatigue resistance.

The hollow stabilizer of this embodiment contains the componentcomposition described above, in which the depth H of the thin portion(trough portion 30 a) of the three-peak shape of the internal weld beadcut portion 30 is 0.3 mm or less and the angle θ formed by the thinportion (trough portion 30 a) and the central thick portions (peakportions 30 b, 30 c) of the three-peak shape is 160° or more and lessthan 180°.

The hardness after the heat treatment is set to high strength of 400 HVor more and less than 580 HV in terms of the Vickers hardness.

Herein, in this embodiment, the “high strength” refers to a case wherethe average hardness in the thickness direction is 400 HV or more andpreferably 450 HV or more in terms of the Vickers hardness. When theaverage hardness in the thickness direction reaches 580 HV or more, areduction in the toughness becomes remarkable, and therefore less than580 HV is set as the upper limit.

The “excellent in fatigue characteristics” as used herein refers to acase where a fatigue test (alternating) with a load stress: ±400 MPa isperformed, so that the number of repetitions before the occurrence of acrack is 200,000 times or more.

EXAMPLE

Next, Example based on this embodiment is described.

The compositions of hot-rolled steel sheets to be used in Example areillustrated in Table 1.

TABLE 1 Steel No. C Si Mn P S Al Ti Cr B N Ca Cu, Ni Nb, V, W REM A 0.400.15 1.25 0.013 0.001 0.035 0.035 0.15 0.0025 0.0021 0.0015 — — — B 0.360.12 1.24 0.015 0.001 0.032 0.034 0.15 0.0022 0.0025 0.0011 — — — C 0.260.11 1.25 0.016 0.001 0.035 0.036 0.15 0.0018 0.0018 0.0013 — — — D 0.220.15 0.55 0.012 0.001 0.036 0.015 0.25 0.0015 0.0022 0.0015 — — — E 0.350.15 1.35 0.015 0.001 0.035 0.035 0.11 0.0012 0.0025 0.0015 Cu 0.15, — —Ni 0.12 F 0.35 0.15 1.35 0.015 0.001 0.035 0.035 0.11 0.0012 0.00250.0015 — Nb 0.015 — G 0.26 0.15 1.35 0.015 0.001 0.035 0.015 0.11 0.00120.0025 0.0015 — V 0.05 H 0.35 0.15 1.35 0.015 0.001 0.035 0.035 0.110.0012 0.0025 0.0015 — W 0.05 I 0.35 0.15 1.35 0.015 0.001 0.035 0.0250.11 0.0012 0.0025 0.0015 — — 0.001 J 0.45 0.12 1.24 0.015 0.001 0.0320.034 0.15 0.0022 0.0025 0.0011 — — — K 0.16 0.12 1.24 0.015 0.001 0.0320.034 0.15 0.0022 0.0025 0.0011 — — — L 0.35 0.12 1.24 0.015 0.001 0.0320.034 0.15 — 0.0025 0.0011 — — — M 0.35 0.12 1.24 0.015 0.001 0.0320.034 — 0.0015 0.0025 0.0011 — — — N 0.35 0.12 1.24 0.015 0.001 0.0320.005 0.12 0.0015 0.0025 0.0011 — — — O 0.36 0.12 1.24 0.015 0.001 0.0320.034 0.15 0.0022 0.0025 — — — — P 0.36 0.12 1.24 0.015 0.012 0.0320.034 0.15 0.0022 0.0025 0.0015 — — —

In Example, a hot-rolled steel sheet (sheet thickness: 4.3 mm) wascontinuously molded using a plurality of rolls by cold rolling to bemade into an open pipe having a substantially cylindrical shape. Next,end portions in the circumferential direction of the open pipe werebutted and pressed into contact with each other, and then subjected toelectric resistance welding using a high frequency electric resistancewelding method to form the electric resistance welded pipe 2 (Outerdiameter of 89.1 mmϕ×Thickness of 4.3 mm).

Furthermore, the obtained electric resistance welded pipe 2 was heatedto a heating temperature (heating temperature before stretch reducing)illustrated in Table 2, and then subjected to a stretch reducing step ofperforming stretch reducing at a cumulative reduction ratio illustratedin Table 2 with a stretch reducer to be made into the electricresistance welded steel pipe 5 of the dimension (Outer diameter of 25.4mmϕ×Thickness of 4.0 mm) illustrated in Table 2. The electric resistancewelded steel pipe 5 was used as a raw material for producing a hollowstabilizer.

Then, in Example, 23 electric resistance welded steel pipes 5 forproducing a hollow stabilizer of Pipe Nos. 1 to 23 were produced asillustrated in Table 2.

From an electric resistance weld zone of each electric resistance weldedsteel pipe 5 for producing a hollow stabilizer, a structure observationtest piece (cross section in which the observation surface isperpendicular to the pipe axis direction) was collected, and then thedepth H of the thin portion (trough portion 30 a) of the three-peakshape of the internal weld bead cut portion 30 of the pipe and the angleθ formed by the thin portion (trough portion 30 a) and the central thickportions (peak portions 30 b, 30 c) of the three-peak shape weremeasured using an optical microscope (magnification: 10 to 20 times).

The electric resistance welded steel pipes 5 for producing a hollowstabilizer were subjected to bending into an L shape by rotary drawbending (with bending radius twice the pipe outer diameter), so thatsamples imitating a hollow stabilizer were produced. At this time, heattreatment including quenching and tempering was applied under theconditions illustrated in Table 2.

The quenching treatment was treatment including performing electricalheating such that the outer surface of the steel pipe had the heatingtemperature illustrated in Table 2, and then immersing the steel pipe ina water tank. After the quenching treatment, tempering treatment ofholding the steel pipe at the temperature illustrated in Table 2 for 20min was applied. Thereafter, shot blasting was applied to the outersurface with a steel ball, so that L-shaped test pieces of Pipe Nos. 1to 23 were produced to be used as samples for a torsional fatigue test.

A hardness measurement piece was collected from each sample before theapplication of the shot blasting, and then hardness measurement wasperformed. The hardness measurement was performed for the cross section(C cross section) perpendicular to the pipe axis direction of the steelpipe and was performed at 0.3 mm pitches from the pipe outer surface tothe pipe inner surface in the thickness direction using a Vickershardness meter (load: 500 gf (4.9 N)).

In the torsional fatigue test, a fatigue test (alternating) with a loadstress: ±400 MPa at a place where the generated stress was the highest(position at about 60° in the pipe circumferential direction from theinner side of the bending) was performed to investigate the number ofrepetitions before the occurrence of a crack. As the test conditions,the load stress was set to ±400 MPa (alternating) and the frequency wasset to 1 Hz.

The obtained results are illustrated in Table 2.

TABLE 2 Heating temperature Internal weld Before reducing After reducingCumulative Thickness before Rolling bead shape Outer Outer reductionreduction stretch finishing Formed Pipe Steel diameter Thicknessdiameter Thickness ratio ratio reducing temperature Depth H angle No.No. (mm) (mm) (mm) (mm) (%) (%) (° C.) (° C.) (mm) (° C.) 1 A 89.1 4.325.4 4 71.5 7 980 830 0.08 172 2 B 89.1 4.3 25.4 4 71.5 7 960 820 0.06175 3 C 89.1 4.3 25.4 4 71.5 7 1000 800 0.05 175 4 D 89.1 4.3 25.4 471.5 7 960 780 0.08 174 5 E 89.1 4.3 25.4 4 71.5 7 980 800 0.09 170 6 F89.1 4.3 25.4 4 71.5 7 970 820 0.1 174 7 G 89.1 4.3 25.4 4 71.5 7 960760 0.08 171 8 H 89.1 4.3 25.4 4 71.5 7 960 800 0.07 170 9 I 89.1 4.325.4 4 71.5 7 920 780 0.06 175 10 J 89.1 4.3 25.4 4 71.5 7 950 800 0.08176 11 K 89.1 4.3 25.4 4 71.5 7 980 790 0.09 177 12 L 89.1 4.3 25.4 471.5 7 980 790 0.08 172 13 M 89.1 4.3 25.4 4 71.5 7 980 850 0.09 171 14N 89.1 4.3 25.4 4 71.5 7 980 750 0.35 140 15 O 89.1 4.3 25.4 4 71.5 7980 750 0.06 170 16 P 89.1 4.3 25.4 4 71.5 7 980 750 0.08 172 17 B 89.13.9 25.4 4 71.4927 −2.6 970 750 0.32 150 18 B 89.1 4.3 17.3 4 80.6 7 960750 0.4 142 19 B 89.1 4.3 25.4 4 71.5 7 1080 860 0.08 171 20 B 89.1 4.325.4 4 71.5 7 980 830 0.08 172 21 B 89.1 4.3 25.4 4 71.5 7 980 830 0.06171 22 B 89.1 4.3 25.4 4 71.5 7 980 830 0.07 170 23 B 89.1 4.3 25.4 471.5 7 860 600 0.09 172 Heat treatment HV Outer Ouenching Temperinghardness surface Fatigue test Pipe electrical-heating temperaturetemperature after heat shot Durable life No. Bending (° C.) (° C.)treatment blasting Remarks (σ = 400 MPa) 1 Cold 900 280 570 Done 350000times bending 2 Cold 900 260 550 Done 310000 times bending 3 Cold 900250 470 Done 280000 times bending 4 Cold 900 300 440 Done 250000 timesbending 5 Cold 900 260 500 Done 300000 times bending 6 Cold 900 260 530Done 310000 times bending 7 Cold 900 260 500 Done 300000 times bending 8Cold 900 260 500 Done 290000 times bending 9 Cold 900 260 500 Done320000 times bending 10 Cold 900 230 590 Done Quenching crack  50000times bending occurred 11 Cold 900 280 380 Done 100000 times bending 12Cold 900 260 380 Done 100000 times bending 13 Cold 900 260 390 Done110000 times bending 14 Cold 900 260 280 Done  70000 times bending 15Cold 900 260 490 Done Crack starting 150000 times bending from inclusion16 Cold 900 260 500 Done Crack starting 110000 times bending frominclusion 17 Cold 900 260 500 Done 150000 times bending 18 Cold 900 260500 Done 140000 times bending 19 Cold 900 260 500 Done High surface120000 times bending decarburization → Surface HV hardness 250 20 Cold1110 260 500 Done High surface 110000 times bending decarburization→Surface HV hardness 230 21 Cold 900 480 330 Done  80000 times bending22 Cold 900 180 500 Done Brittle fracture  80000 times bending 23 Cold900 260 500 Done Crack in Untestable bending cold work

As is understood from Table 2, in Pipe Nos. 1 to 9 based on aspects ofthe present invention, the hardness after the quenching and temperingtreatment can be set to 400 HV or more and the strength is high (highhardness). Furthermore, the number of repetitions of loading before afracture by the fatigue test is 200,000 times or more. Thus, it is foundthat a high strength hollow stabilizer excellent in fatiguecharacteristics can be produced.

Meanwhile, a comparison between Pipe No. 2 and Pipe Nos. 17 and 18 showsthat, even when the hardness after the heat treatment is set so that thestrength is as high as 500 HV or more in all the pipes with use of thesame steel material, Pipe Nos. 17 and 18 in which the shape of theinternal bead cut portion does not satisfy the range according toaspects of the present invention have the number of repetitions ofloading before a fracture by the fatigue test of less than 200,000times. Thus, it is found that, by forming the shape of the internal beadcut portion to satisfy the range according to aspects of the presentinvention, a hollow stabilizer excellent in fatigue characteristics canbe produced when the strength is set to the same high strength.

Herein, in Pipe No. 10, the strength was set to 590

HV, and therefore the toughness decreased, so that a crack occurred.More specifically, in Pipe No. 10, due to an attempt to forciblyincrease the hardness of the end product to 590 HV, a strain due to aquenching crack was formed in the product, so that the life wasshortened.

In Pipe Nos. 15, 16, due to the fact that an excessive amount of S asthe material was compounded or Ca was not compounded, a crack startingfrom an inclusion occurred.

Pipe Nos. 11 to 14 have hardness of less than 400 HV and do not achievehigh strength, and therefore the number of repetitions of loading beforea fracture by the fatigue test is less than 200,000 times. However, byforming the shape of the internal bead cut portion to satisfy the rangeaccording to aspects of the present invention, the fatiguecharacteristics are improved as compared with a case where the shape ofthe internal bead cut portion is not formed to satisfy the rangeaccording to aspects of the present invention.

As described above, by forming the shape of the three-peak shape of theinternal bead cut portion of the electric resistance welded steel pipe 5for producing a hollow stabilizer in which the shape of the internalbead cut portion is the three-peak shape by the application of the hotstretch reducing to satisfy the range according to aspects of thepresent invention, the fatigue characteristics of the electricresistance welded steel pipe 5 for producing a hollow stabilizer can beimproved.

Furthermore, it is found that, when a hollow stabilizer having hardnessof 400 HV or more and less than 580 HV is produced using the electricresistance welded steel pipe 5 for producing a hollow stabilizer, a highstrength hollow stabilizer excellent in fatigue characteristics can beproduced.

As described above, the entire contents of Japanese Patent ApplicationNo. 2018-065875 (filed Mar. 29, 2018) to which this application claimspriority form part of this disclosure by reference. Although thedescription is given referring to a limited number of embodimentsherein, the scope of the present invention is not limited thereto. It isobvious for those skilled in the art to alter and modify the embodimentsbased on the disclosure above.

REFERENCE SIGNS LIST

-   -   1 first step    -   1A open pipe molding unit    -   1B electric resistance welding treatment unit    -   1C bead cutting treatment unit    -   2 electric resistance welded pipe    -   3 second step    -   3A heat treatment unit    -   3B hot stretch reducing treatment unit    -   4 third step    -   5 electric resistance welded steel pipe    -   12 weld line    -   30 internal weld bead cut portion    -   30 a trough portion    -   30 b, 30 c peak portion    -   40 hollow stabilizer    -   H depth    -   θ angle

1. An electric resistance welded steel pipe for producing a hollowstabilizer, the electric resistance welded steel pipe being a steel pipefor producing a hollow stabilizer and comprising: on a pipe innersurface, an internal weld bead cut portion having an outline shape of athree-peak shape along a circumferential direction in which a troughportion with a reduced thickness is formed on each of both right andleft sides across a weld zone, wherein in the internal weld bead cutportion, a depth H of the trough portion is 0.3 mm or less and an angleθ formed by a central portion in the circumferential direction of thetrough portion and a top of right and left peak portions located on bothright and left sides of the trough portion is 160° or more and less than180°.
 2. The electric resistance welded steel pipe for producing ahollow stabilizer according to claim 1, wherein a steel materialconfiguring the steel pipe contains, in terms of % by mass, C: 0.20% ormore and 0.40% or less, Si: 0.1% or more and 1.0% or less, Mn: 0.1% ormore and 2.0% or less, P: 0.100% or less, S: 0.010% or less, Al: 0.01%or more and 0.10% or less, Cr: 0.01% or more and 0.50% or less, Ti:0.01% or more and 0.05% or less, B: 0.0005% or more and 0.0050% or less,Ca: 0.0001% or more and 0.0050% or less, N: 0.0050% or less, and abalance of Fe and an inevitable impurity.
 3. The electric resistancewelded steel pipe for producing a hollow stabilizer according to claim2, wherein the steel material configuring the steel pipe furthercontains, in terms of % by mass, either or both of Cu: 0.05% or more and1.00% or less and Ni: 0.05% or more and 1.00% or less.
 4. The electricresistance welded steel pipe for producing a hollow stabilizer accordingto claim 2, wherein the steel material configuring the steel pipefurther contains, in terms of % by mass, one or two or more kindsselected from Nb: 0.001% or more and 0.050% or less, W: 0.001% or moreand 0.050% or less, and V: 0.05% or more and 0.50% or less.
 5. Theelectric resistance welded steel pipe for producing a hollow stabilizeraccording to claim 2, wherein the steel material configuring the steelpipe further contains, in terms of % by mass, REM: 0.001% or more and0.020% or less.
 6. A hollow stabilizer comprising: the electricresistance welded steel pipe for producing a hollow stabilizer accordingto claim 1, wherein Vickers hardness is 400 HV or more and less than 580HV.
 7. A production method for a hollow stabilizer comprising: applyingcold bending to the electric resistance welded steel pipe for producinga hollow stabilizer according to claim 1; applying heat treatmentincluding quenching treatment or quenching and tempering treatment tothe electric resistance welded steel pipe; and adjusting hardness afterthe heat treatment to 400 HV or more and less than 580 HV in terms ofVickers hardness.
 8. A production method for an electric resistancewelded steel pipe for producing a hollow stabilizer comprising: a firststep of molding a steel sheet into a cylindrical shape by cold moldingto form an open pipe, butting end portions in a width direction of theopen pipe against each other, and performing electric resistance weldingto the end portions to form an electric resistance welded pipe; a secondstep of heating the electric resistance welded pipe to a temperature of850° C. or more and 1000° C. or less, and then applying hot stretchreducing to the electric resistance welded pipe at a rolling temperatureof 650° C. or more and 1000° C. or less and at a cumulative reductionratio of 30% or more and 90% or less; and a third step of adjusting aninternal weld bead cut portion having an outline shape of a three-peakshape along a circumferential direction in which a trough portion with areduced thickness is formed on each of both right and left sides acrossa weld zone such that a depth H of the trough portion is 0.3 mm or lessand an angle θ formed by a central portion in the circumferentialdirection of the trough portion and a top of right and left peakportions located on both right and left sides of the trough portion is160° or more and less than 180°.
 9. The production method for anelectric resistance welded steel pipe for producing a hollow stabilizeraccording to claim 8, wherein the steel sheet contains, in terms of % bymass, C: 0.20% or more and 0.40% or less, Si: 0.1% or more and 1.0% orless, Mn: 0.1% or more and 2.0% or less, P: 0.100% or less, S: 0.010% orless, Al: 0.01% or more and 0.10% or less, Cr: 0.01% or more and 0.50%or less, Ti: 0.01% or more and 0.05% or less, B: 0.0005% or more and0.0050% or less, Ca: 0.0001% or more and 0.0050% or less, N: 0.0050% orless, and a balance of Fe and an inevitable impurity.
 10. The productionmethod for an electric resistance welded steel pipe for producing ahollow stabilizer according to claim 9, wherein the steel sheet furthercontains, in terms of % by mass, one or two or more kinds selected fromCu: 0.05% or more and 1.00% or less, Ni: 0.05% or more and 1.00% orless, Nb: 0.001% or more and 0.050% or less, W: 0.001% or more and0.050% or less, V: 0.05% or more and 0.50% or less, and REM: 0.001% ormore and 0.020% or less.
 11. The electric resistance welded steel pipefor producing a hollow stabilizer according to claim 3, wherein thesteel material configuring the steel pipe further contains, in terms of% by mass, one or two or more kinds selected from Nb: 0.001% or more and0.050% or less, W: 0.001% or more and 0.050% or less, and V: 0.05% ormore and 0.50% or less.
 12. The electric resistance welded steel pipefor producing a hollow stabilizer according to claim 2, wherein thesteel material configuring the steel pipe further contains, in terms of% by mass, REM: 0.001% or more and 0.020% or less.
 13. The electricresistance welded steel pipe for producing a hollow stabilizer accordingto claim 3, wherein the steel material configuring the steel pipefurther contains, in terms of % by mass, REM: 0.001% or more and 0.020%or less.
 14. The electric resistance welded steel pipe for producing ahollow stabilizer according to claim 4, wherein the steel materialconfiguring the steel pipe further contains, in terms of % by mass, REM:0.001% or more and 0.020% or less.
 15. A hollow stabilizer comprising:the electric resistance welded steel pipe for producing a hollowstabilizer according to claim 2, wherein Vickers hardness is 400 HV ormore and less than 580 HV.
 16. A hollow stabilizer comprising: theelectric resistance welded steel pipe for producing a hollow stabilizeraccording to claim 3, wherein Vickers hardness is 400 HV or more andless than 580 HV.
 17. A hollow stabilizer comprising: the electricresistance welded steel pipe for producing a hollow stabilizer accordingto claim 4, wherein Vickers hardness is 400 HV or more and less than 580HV.
 18. A hollow stabilizer comprising: the electric resistance weldedsteel pipe for producing a hollow stabilizer according to claim 5,wherein Vickers hardness is 400 HV or more and less than 580 HV.
 19. Aproduction method for a hollow stabilizer comprising: applying coldbending to the electric resistance welded steel pipe for producing ahollow stabilizer according to claim 2; applying heat treatmentincluding quenching treatment or quenching and tempering treatment tothe electric resistance welded steel pipe; and adjusting hardness afterthe heat treatment to 400 HV or more and less than 580 HV in terms ofVickers hardness.
 20. A production method for a hollow stabilizercomprising: applying cold bending to the electric resistance weldedsteel pipe for producing a hollow stabilizer according to claim 3;applying heat treatment including quenching treatment or quenching andtempering treatment to the electric resistance welded steel pipe; andadjusting hardness after the heat treatment to 400 HV or more and lessthan 580 HV in terms of Vickers hardness.